1. Field of the Invention
The present invention relates to an electroluminescence (hereinafter simply referred to as “EL”) display circuit for controlling light emission from an EL element based on both a data voltage and data current generated based on data for driving the EL element.
2. Description of the Prior Art
Because EL display devices in which a self-emitting EL element is used as an emissive element in each pixel have advantages such as that the device is thin, self-emitting, and consumes less power, EL display devices have attracted much attention as alternatives to display devices such as liquid crystal display (LCD) and cathode ray tube (CRT) display devices.
In particular, a high resolution display can be achieved by an active matrix EL display device in which a switching element such as a thin film transistor (hereinafter simply referred to as “TFT”) for individually controlling an EL element is provided in each pixel and the EL element in each pixel is controlled.
In an active matrix EL display device, a plurality of gate lines extend along a row direction over a substrate, a plurality of data lines and power supply lines extend along a column direction over the substrate, and each pixel has an organic EL element, a selection TFT, a driver TFT, and a storage capacitor. In this structure, a gate line is selected so that the selection TFT is switched on, a data voltage on a data line is charged into the storage capacitor, and the driver TFT is switched on by this data voltage to allow electric power to flow from a power supply line through the organic EL element.
Japanese Patent Laid-Open Publication No. 2001-147659 (hereinafter simply referred to as “the '659 Publication”) discloses a circuit in which two p-channel TFTs are added in each pixel as controller transistors and a data current corresponding to display data is applied to a data line.
FIG. 1 shows a pixel circuit disclosed in the '659 Publicatio. As shown, one terminal of an n-channel TFT3 (selection TFT) having its gate connected to a scan line scanA is connected to a data line DL onto which a current Iw is to be applied. The other terminal of the selection TFT3 is connected to one terminal of a p-channel TFT1 and one terminal of a p-channel TFT4 (driver TFT). The other terminal of the TFT1 is connected to a power supply line Vdd and a gate of the TFT1 is connected to a gate of a p-channel TFT2 for driving an organic EL element (“OLED”). The other terminal of the TFT4 is connected to the gates of the TFT1 and TFT2 and a gate of the TFT4 is connected to a scan line scanB.
In this structure, the scan line scanA is set to an H level to switch the TFT3 on, and the scan line scanB is set to an L level to switch on the TFT4. A current Iw corresponding to data is applied to the data line DL, which causes the gate and source of the TFT1 to be connected (short-circuited) due to the switching on of the TFT4, the current Iw is converted to a voltage, and this voltage is set to voltages of the gates of the TFT1 and TFT2. After the TFT3 and TFT4 are switched off, the gate voltage of the TFT2 is maintained by a storage capacitor C, thus allowing a current corresponding to the current Iw to flow through the TFT2 and through the OLED so that light is emitted from the OLED based on the amount of supplied current. Then, when the scanB is set to an L level, the TFT1 is switched on, the gate voltage of the TFT1 is increased, the storage capacitor C is discharged, data is erased, and the TFT1 and TFT2 are switched off.
In this circuit, when a current flows through the TFT1, the current is converted to a voltage and the gate voltage of the TFT1 and TFT2 is determined. According to the determined gate voltage, the amount of current flowing through the TFT2 is determined. Thus, the amount of current flowing through the TFT2 can be set corresponding to a data current Iw.
However, in the circuit of the '659 Publicatio, the data current Iw is allowed to flow through the TFT1 to set the gate voltage of the TFT2. Therefore, it cannot be assured that the current flowing through the TFT2 corresponds to the data current. Thus, this system is commonly called an “indirect specification system”.
Another reference, R. Hattori et al., IECE TRANS. ELECTRON., Vol. E83-C, No. 5, pp. 779-782, May (2000) (hereinafter simply referred to as the “Hattori reference”) discloses a circuit having a structure in which a data current is set to a storage capacitor while the data current is supplied onto the data line and flows through a driver TFT. Because a gate voltage of the driver TFT is directly determined by the data current, this system is commonly referred to as “direct specification system”.
FIG. 2 shows a structure of a circuit disclosed in the Hattori reference. A source of a p-channel driver TFT5 is connected to a power supply Vdd, an anode of an organic EL element OLED is connected to a drain of the driver TFT5 through a p-channel TFT6, and a cathode of the OLED is connected to a ground.
A gate of the driver TFT5 is connected to a data line DL through a p-channel TFT7 and is connected to a power supply line Vdd through a storage capacitor C. In addition, a connection point between the driver TFT5 and the TFT6 is connected to the data line DL through a TFT8.
A read line Read which extends along the row direction is connected to a gate of the TFT6 and a write line Write which also extends along the row direction is connected to gates of the TFT7 and TFT8.
In this circuit, while a data current corresponding to display data is supplied onto the data line DL, the write line Write is set to an L level to switch on the TFT7 and TFT8 and the read line Read is set to an H level to switch off the TFT6. With this configuration, a data current Idata flowing on the data line DL flows from the power supply Vdd through the driver TFT5 and TFT8. Because TFT7 is switched on, the gate voltage of the TFT5 is set to a voltage of the TFT5 when Idata flows through the TFT5 and this voltage is stored in the storage capacitor C.
Then, the write line Write is set to an H level and the read line Read is set to an L level to switch off the TFT7 and TFT8 and switch on the TFT6. Because the gate voltage of the TFT5 is maintained at the voltage stored in the storage capacitor C, a current identical to the current Idata continues to flow through the TFT5.
In this manner, a current Ioled corresponding to the data current Idata can flow through the organic EL element OLED and light can be emitted. In particular, in this circuit, a data voltage is written into the storage capacitor C by actually supplying the data current Idata corresponding to the display data through the driver TFT5. With this structure, it is possible to precisely set the drive current Ioled of the organic EL element OLED.
As described, with the direct specification system, it is possible to more precisely control a drive current of an organic EL element.
In such a circuit, however, a current value corresponding to minimum video data (minimum current value) is directly written into the storage capacitor. When the number of gradations is small, it is possible to set the minimum current value to a relatively large value. However, when it is desired that the number of gradations be large in order to realize a high resolution display, the minimum current value is significantly small. In order to reliably set a charge voltage of the storage capacitor corresponding to a data current having a small current value, the time required for writing data for each pixel becomes significantly large. Therefore, in a direct specification system, there had been a problem in that a display with a large number of pixels and a large number of gradations was difficult.
In the indirect specification system, on the other hand, it is possible to set the write current corresponding to the minimum video data to a relatively large value by changing the sizes (size ratio) of the TFT1 and TFT2, which allows for a short writing time. However, as described above, the indirect specification system is inferior to the direct specification system in the precision of written data.